Epoxy resin matrices are usually two-component systems, comprising a resin component [e.g. a diglycidyl ether of bisphenol A (DGEBA)] and a crosslinking agent [e.g. methylenedianiline (MDA)]. Diluents or fillers are also often added for economic or processing reasons. The resin and curing agent are mixed and "cured", usually at elevated temperatures. The above matrix would typically have a strength of 85 MPa, a modulus of 2800 MPa, a glass transition temperature (T.sub.g, a maximum use temperature) of .about.150.degree. C. and would fracture in an essentially brittle fashion (H. Lee and K. Neville, "Handbook of Epoxy Resins", McGraw-Hill, New York, 1967). Considerable effort has been expended to improve on these properties.
However, no completely satisfactory solution currently exists because improvement in one property has been gained only at the expense of another (e.g. solvent resistance, T.sub.g, ductility, processability, cost).
Many curing agents or hardeners have been used in epoxy resin systems, in particular amines such as methylene dianiline and tetraethylenetriamine. In some cases, adducts of these same amines with mono- or diepoxides have been used as curing agents. In U.S. Pat. No. 4,182,831. Jan. 89, 1980, Hicks, polyepoxide resins are reacted with an aliphatic primary amine mixture, then further reacted with a monoepoxide. U.S. Pat. No. 4,182,832 Jan. 8, 1980, Zondler et al, describes as curing agents for epoxy resins, adducts of substituted 1,4-diaminobutanes and liquid epoxides. U.S. Pat. No. 4,195,152, Mar. 25, 1980, Floyd, describes an adduct of N-alkylpolyamines and mono-epoxides as epoxy resin curing agents. U.S. Pat. No. 4,197,389, Apr. 8, 1980, Becker et al, describes a hardening agent (curing agent) for epoxy resins which is a reaction product of an aliphatic or heterocyclic polyamine, a polyalkylene polyether polyol and at least one polyepoxide. These patents are typical of the more complex curing agents being developed.
Antiplasticization in highly crosslinked epoxy resin systems has been reported in the literature. For instance, N. Hata et al, in J. Appl. Polym. Sci., 17(7), p. 2173-81, 1973, describe pentachlorobiphenyl as the most effective antiplasticizer, and dibutylphthalate and 2,2-bis-[4-(2-hydroxy-3-phenoxypropoxy)phenyl] propane as less effective antiplasticizers. There evidently has been little recent interest in this, possibly because materials such as pentachlorobinphenyl are environmentally objectionable.
In U.S. Pat. No. 3,449,375, June 10, 1969, Newey, flame-retardant epoxy resins are described comprising the reaction product of a haloalkyl monoepoxide and a trihaloaniline, which is dehydrohalogenated to form a diepoxide having two epoxyalkyl groups attached to the amino group. The reaction is controlled so that the epoxide groups remain unreacted, and no hydroxyl groups are formed. These trihalodiglycidyl anilines exhibit reactions typical of the glycidyl ether group and may be added to other epoxy resins and the mixture cured to give strong, flame-retardant products. One of the other epoxy resin materials may be a monomeric polyepoxide such as vinyl cyclohexene dioxide. It should be noted that the latter diepoxide is not reacted with the trihaloaniline itself in this Newey patent.
We have found an alternative method of improving the properties of epoxy resins, by adding reaction products which act as fortifiers, to conventional epoxy resin+curing agent formulations.